Thermoelectric cooling devices which are based on the Peltier effect allow cooling below ambient temperature and are more compact and less expensive than comparable cooling systems. Moreover, they have no moving parts and do not require any maintenance. One major drawback, however, is that the cooling efficiency of state of the art thermoelectric coolers is rather low and that they dissipate a lot of heat which reduces the overall performance considerably.
The performance and efficiency of a thermoelectric module can directly be related to its thermoelectric figure of merit zT as well as to its temperature gradient ΔT which is defined by the two temperatures Th and Tc at the hot and the cold side of the module, respectively. The thermoelectric figure of merit, which describes the ability of a material to convert heat into electricity, is given by
      zT    =                            S          2                ·        σ        ·        T            κ        ,where S is the Seebeck coefficient, a the electrical conductivity, T the absolute temperature, and κ the thermal conductivity. The overall thermal conductivity κ is composed by the sum of the contributions of the electronic and lattice thermal conductivity κe and κl, respectively. Thus, the performance of a thermoelectric module can be increased by maximizing the so-called power factor S2·τ or by minimizing the thermal conductivity κ=κe+κl. Thus, in order to achieve a high zT, the system should be a good conductor for electrons but a bad conductor for phonons. The latter also implies a large ΔT throughout the material since a large thermal conductivity would short the thermal circuit.
Prior art thermoelectric modules used for cooling purposes exhibit a rather low efficiency with a figure of merit zT lying around unity. The reason is that the Seebeck coefficient and the electrical conductivity are interrelated in a way that if one tries to increase the electrical conductivity by increasing the charge carrier density, e.g. with heavily doping the material with appropriate elements, the Seebeck coefficient decreases. Hence, improving the figure of merit zT in thermoelectric materials is still a big challenge in material science, hampering the development of novel, more efficient thermoelectric cooling devices.
Macroscopically, the heat transfer in a standard thermoelectric cooler is governed by the current I which is driven from one side of the device to the other. Beside the low figure of merit, the heat transfer in state of the art thermoelectric materials is also rather inefficient due to heat dissipation at elevated operation current I, such that the thermoelectric cooler consumes more power than it effectively transports. Actual cooling modules roughly consume twice as much energy (in the form of electricity) as they transport (in the form of heat). The effective cooling is thereby massively reduced by losses associated with electrical resistance in form of Joule heating and thermal conductivity. The former causes internal heating through I2R losses. This is a very critical issue because it imposes a lot more heat on the heat sink to cool. Hence, the temperature on the hot side of the cooler, which is connected to the heat sink, is higher so that the effective cooling temperature achieved through the cooling process at the cold side TC given by ΔT=Th−TC is also higher.
Beside the intrinsic thermal conductivity of the thermoelectric material which is a limiting factor of the maximal ΔT, ΔT is thus greatly determined by the total amount of power transferred by the thermoelectric module. For instance, if 10 W of heat is transferred across the thermoelectric module from the cold side to the hot side, another 20 W may be added due to Joule heating, so that overall 30 W are transferred to the heat sink. As a consequence, the maximal ΔT may be reduced e.g. from about 60 K to only 20 K due to I2R losses. This illustrates that prior art thermoelectric cooling devices add a lot of excess heat to the system, which considerably reduces the performance of the overall cooling process.
US 2015/0155464 A1 discloses a thermoelectric element comprising a plurality of alternatingly disposed n-type and p-type structures which are electrically connected in series by a plurality of electrodes. The electrodes are in contact with one of two thermally conductive plates. Each of the structures is made of a topological insulator material which may be altered by adding chemical dopants in order to tune its Fermi level. The thermoelectric element may be used for Peltier cooling or for power generation.